Forwarder: propagate the enveloped message's Context (#387)

This may be useful if the Forwarder's router middleware changes the
Context in some way.

components/forwarder/envelope.go

@@ -69,6 +69,7 @@ func unwrapMessageFromEnvelope(msg *message.Message) (destinationTopic string, u
 
  watermillMessage := message.NewMessage(envelopedMsg.UUID, envelopedMsg.Payload)
  watermillMessage.Metadata = envelopedMsg.Metadata
+ watermillMessage.SetContext(msg.Context())
 
  return envelopedMsg.DestinationTopic, watermillMessage, nil
 }